1. Field of the Invention
The present invention relates to a method and apparatus for preparing injection molding compound pellets for use and, more particularly, to a method and apparatus for cleaning pellets and separating out broken pellets prior to use in an automatic molding machine.
2. Description of the Related Art
Injection molding, which includes extremely high pressure processes used for article molding, as well as transfer molding used in semiconductor processing, is a commonly used manufacturing process. Traditionally, this process has been a manual operation in which a machine operator handles a manual molding press. This press is manipulated to inject a raw plastic pellet/tablet into the veins and cavities of a die set which is a template image of the end product. The operator causes injection by allowing a plunger, powered by a hydraulic ram, to compress the compound through a cylinder into the cavities of a die set at high temperature and pressure. The high temperature and pressure conditions allow the compound to flow into the cavities of the die set where the compound is later cooled and released as an end product. This manual molding press requires the operator to place pellets into the machine and operate the hydraulic ram for each machine cycle to make the end product.
More recently, automated molding equipment has been introduced into manufacturing. Using this equipment, the operator does not need to manually place the pellets into the machine and operate the press during each cycle. The most automated of this equipment only requires that the operator place a batch of the compound, usually in a pellet form, into a hopper. This hopper transfers the molten pellet material into a molding die set which automatically forms each batch of end product through a sequence of mechanical steps which is typically controlled by a computer. This automatic operation, however, is not free from problems.
Two types of reliability problems are inherent to automated molding machines that process compound pellets. First, automated molding machines commonly utilize automatic pellet loading equipment that uses sensors to detect the presence of pellets as it cycles them through the molding machine. Automated molding machines frequently jam when broken pellets are introduced into the loading area. These pellets are commonly broken during shipping or handling. They end up in the pellet hopper because of the operator's failure to remove them. Abnormal pellets cause the machine to jam because the automatic pellet loading equipment uses sensors, such as pattern detectors, that have difficulty detecting pellets of an abnormal shape or size.
A second type of problems arise in automated molding machines due to the use of sensors for detecting positioning of automated assemblies within the machine. The automated molding machines use electronic sensors such as electronic eyes to detect the positioning of these various assemblies. If the electronic sensors fail to detect the positioning of an assembly within the machine, the machine generally enters a failure mode, stopping its operation and causing jamming. Sensors become blocked or fouled with mold compound dust that originates from the mold compound pellets.
Reliability problems in these two areas cause a significant degradation of the manufacturing process because they usually result in substantial equipment stoppages or jamming that ultimately reduces equipment utilization and depresses productivity. What is sought is a method and apparatus that will substantially decrease the occurrence of stoppages on these machines. The previous method of operation, depicted in FIG. 1, requires an operator to perform a sequence of manual steps to clean and sort pellets before they used in an automatic molding machine. In a first step 100 involving manual handling of the pellets, the operator moves pellets 80 from their shipping box 82 and loads them into a sorting box 84. In a second step requiring manual handling of the pellets, the operator visually inspects the pellets in the sorting box 84 to remove broken pellets and excessive dust. Then the operator uses a scoop 86 to transfer the pellets from the box 84 into the machine hopper 88 in a third step 104 in which the pellets are handled. This hopper transfers the pellets into the automatic molding machine.
With the advent of costly automated molding injection machines, the minimization of molding machine downtime is of increased performance. The condition of the molding compound pellets that are fed into the machines is critical for productive use of the machines. Automated molding injection machines having automatic loaders require that the pellets be geometrically true and free from excessive dust. What is desired is a reliable, rapid, easy to operate and inexpensive apparatus and method for separating out broken pellets. What is also sought is a reliable, rapid, easy to operate and inexpensive apparatus and method that removes excessive dust from the pellets. By removing geometrically bad pellets and excessive dust from the pellets, the propensity of the automated molding injection machines to jam at the automatic loader is greatly reduced. In addition, reducing the amount of dust from the pellets helps to avoid machine failures caused by dusty sensors such as robotic electronic eyes in mechanical feed assemblies.